1. Field of the Invention
The present invention relates to a turbo-molecular pump for evacuating gas with a rotor that rotates at a high speed, and more particularly to a turbo-molecular pump having a radial turbine blade pumping section in a casing.
2. Description of the Related Art
FIG. 12 of the accompanying drawings shows a conventional turbo-molecular pump having a radial turbine blade pumping section in a casing. As shown in FIG. 12, the conventional turbo-molecular pump comprises a rotor R and a stator S which are housed in a casing 10. The rotor R and the stator S jointly make up an axial turbine blade pumping section L1 and a radial turbine blade pumping section L2. The stator S comprises a base 14, a stationary cylindrical sleeve 16 vertically mounted centrally on the base 14, and stationary components of the axial turbine blade pumping section L1 and the radial turbine blade pumping section L2. The rotor R comprises a main shaft 18 inserted in the stationary cylindrical sleeve 16, and a rotor body 20 fixed to the main shaft 18.
Between the main shaft 18 and the stationary cylindrical sleeve 16, there are provided a drive motor 22, and upper and lower radial bearings 24 and 26 provided above and below the drive motor 22. An axial bearing 28 is disposed at a lower portion of the main shaft 10, and comprises a target disk 28a mounted on the lower end of the main shaft 18, and upper and lower electromagnets 28b provided on the stator side. Further, touchdown bearings 29a and 29b are provided at upper and lower portions of the stationary cylindrical sleeve 16.
With this arrangement, the rotor R can be rotated at a high speed under 5-axis active control. The rotor body 20 in the axial turbine blade pumping section L1 has disk-like rotor blades 30 integrally provided on an upper outer circumferential portion thereof. In the casing 10, there are provided stator blades 32 disposed axially alternately with the rotor blades 30. Each of the stator blades 32 has an outer edge clamped by stator blade spacers 34 and is thus fixed. Each of the rotor blades 30 has a wheel-like configuration which has a hub at an inner circumferential portion thereof, a frame at an outer circumferential portion thereof, and inclined blades (not shown) provided between the hub and the frame and extending in a radial direction. Thus, the turbine blades 30 are rotated at a high speed to make an impact on gas molecules in an axial direction for thereby evacuating gas.
The radial turbine blade pumping section L2 is provided downstream of, i.e. below the axial turbine blade pumping section L1. In the radial turbine blade pumping section L2, the rotor body 20 has disk-like rotor blades 36 integrally provided on an outer circumferential portion thereof in the same manner as the axial turbine blade pumping section L1. In the casing 10, there are provided stator blades 38 disposed axially alternately with the rotor blades 36. Each of the stator blades 38 has an outer edge clamped by stator blade spacers 40 and is thus fixed.
Each of the stator blades 38 is in the form of a follow disk, and as shown in FIGS. 13A and 13B, each of the stator blades 38 has spiral ridges 46 which are formed in the front and backside surfaces thereof and extend between a central hole 42 and an outer circumferential portion 44, and spiral grooves 48 whose widths are gradually broader radially outwardly and which are formed between the adjacent ridges 46. The spiral ridges 46 on the front surface, i.e. upper surface of the stator blade 38 are configured such that when the rotor blade 36 is rotated in a direction shown by an arrow A in FIG. 13A, gas molecules flow inwardly as shown by a solid line arrow B. On the other hand, the spiral ridges 46 on the backside surface, i.e. lower surface of the stator blade 38 are configured such that when the rotor blade 36 is rotated in a direction shown by the arrow A in FIG. 13A, gas molecules flow outwardly as shown by a dotted line arrow C. Each of the stator blade 38 is usually composed of two half segments, or three or more divided segments. The stator blades 38 are assembled by interposing the stator blade spacers 40 so that the stator blades 38 alternate with the rotor blades 36, and then the completed assembly is inserted into the casing 10.
With the above configuration, in the radial turbine blade pumping section L2, a long evacuation passage extending in zigzag from top to bottom between the stator blades 38 and the rotor blades 36 is constructed within a short span in the axial direction, thus achieving high evacuation and compression performance without making the radial turbine blade pumping section L2 long in the axial direction.
In the radial turbine blade pumping section L2, the outer diameter D1 of the rotor at its portion facing the inner circumferential surface of the stator blade 38 is set to the same dimension in all stages, and the inner diameter D2 of the stator (outer diameter of the spiral ridge-groove section) at its portion facing the outer circumferential surface of the rotor blade 36 is set to the same dimension in all stages.
However, in the case of the conventional turbo-molecular pump having the radial turbine blade pumping section L2, as shown in FIG. 14, the gap G1 between the stator blade 38 located at the first stage in the radial turbine blade pumping section L2 and the rotor blade 30 located immediately above this first-stage stator blade 38 and at the lowermost stage in the axial turbine blade pumping section L1 is constant. Therefore, the cross-sectional area of the flow passage extending along the upper surface of the stator blade 38 toward the inner circumferential side of the stator blade 38, i.e. the inner circumferential side of the radial turbine blade pumping section L2 decreases drastically in proportion to the radius of the stator blade 38. Consequently, the gas is prevented from flowing smoothly to the inner circumferential side of the radial turbine blade pumping section L2 to cause stagnation of the gas. Further, when the gas turns its flow direction from the axial direction to the radial direction, the gas cannot be smoothly flowed to be stagnated, thus lowering the evacuation performance of the pump.
The present invention has been made in view of the above drawbacks in the conventional turbo-molecular pump. It is therefore an object of the present invention to provide a turbo-molecular pump which can create smooth gas flow therein and prevent the evacuation performance from lowering.
According to a first aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein at least one of the stator blade and the rotor blade which are located at a first stage of the radial turbine blade pumping section has such a shape that the at least one of the stator blade and the rotor blade is smaller in thickness in a direction of gas flow.
With the above arrangement, at least one of the cross-sectional area of the flow passage defined between the stator blade at the first stage in the radial turbine blade pumping section and the rotor blade located immediately above this first-stage stator blade and at the lowermost stage in the axial turbine blade pumping section and the cross-sectional area of the flow passage defined between the rotor blade at the first stage in the radial turbine blade pumping section and the stator blade located immediately above this first-stage rotor blade and at the lowermost stage in the axial turbine blade pumping section is prevented from being drastically smaller in the direction of gas flow. Thus, the gas flowing from an upstream side into the radial turbine blade pumping section can be guided smoothly toward the inner circumferential side of the radial turbine blade pumping section.
According to a second aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein an outer diameter of the rotor at its portion facing an inner circumferential surface. of a stator blade at a first stage in the radial turbine blade pumping section is smaller than an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at any one of stages subsequent to the first stage.
With this arrangement, the cross-sectional area of the flow passage in an axial direction defined between the inner circumferential surface of the stator blade at the first stage and the outer circumferential surface of the rotor at its portion facing the inner circumferential surface of this first-stage stator blade is enlarged for thereby guiding the gas toward a radial direction in flow passages upstream and downstream of the flow passage in the axial direction.
According to a third aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein one of an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at a first stage in the radial turbine blade pumping section is larger than an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at any one of stages subsequent to the first stage.
With this arrangement, the cross-sectional area of the flow passage in an axial direction defined between the outer circumferential surface of the rotor blade at the first stage and the inner circumferential surface of the stator at its portion facing the outer circumferential surface of this first-stage rotor blade or the outer diameter of the spiral ridge-groove section is enlarged for thereby guiding the gas toward a radial direction in flow passages upstream and downstream of the flow passage in the axial direction. Generally, the inner circumferential surface of the stator at its portion facing the outer circumferential surface of this first-stage rotor blade and the outer diameter of the spiral ridge-groove section have the same dimension.
According to a fourth aspect of the present invention, there is provided a turbo-molecular pump comprising: a casing; a stator fixedly mounted in the casing and having stator blades; a rotor rotatably provided in the casing and having rotor blades, the rotor blades alternating with the stator blades; and a radial turbine blade pumping section having a spiral ridge-groove section provided on at least one of surfaces, facing each other, of the stator blade and the rotor blade; wherein an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at a first stage in the radial turbine blade pumping section is smaller than an outer diameter of the rotor at its portion facing an inner circumferential surface of a stator blade at any one of stages subsequent to the first stage; one of an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at a first stage in the radial turbine blade pumping section is larger than an inner diameter of the stator and an outer diameter of the spiral ridge-groove section at its portion facing an outer circumferential surface of a rotor blade at any one of stages subsequent to the first stage.
The above and other objects, features, and advantages of the present invention will be apparent from the following description when taken in conjunction with the accompanying drawings which illustrates preferred embodiments of the present invention by way of example.